[COPYRIGHTED]

U. S. NAVAL INSTITUTE, ANNAPOLIS, MD.

RADIO EQUIPMENT ON NC SEAPLANES By LIEUT. COMMANDER ROBERT A. Lavender, U. S. Navy

The prominent part taken by radio apparatus in the transAtlantic flight of the NC seaplanes has emphasized the uses and needs of such radio apparatus during long flights along the coast and the absolute necessity of efficient apparatus within reasonable limits of weight for the commercial trans-Atlantic air lines which will be organized in the future. It is a curious fact, but yet to be expected, that when the designers and the pilots of air-craft think of radio sets, they invariably think in pounds. Pounds of weight and pounds of head resistance mean pounds of gasoline and oil. And pounds of gasoline and oil mean speed and endur

But what good would extra pounds of gasoline and oil do if communication and navigation were made uncertain and the lives of the crew and passengers jeopardized when lost during a fog or when drifting on the water with no means of communication? The 26 pounds of radio equipment (emergency transmitter) removed from the NC-3 at Trepassey Bay, because the plane could not get off the water with full load, should have remained on board for it would have been worth its weight in gold when the NC-3 came down on the water, a few hours later, 50 miles south of the Azores, with 160 gallons of gasoline, but with no means of communication.

When the radio apparatus was selected for the NC seaplanes, the greatest consideration was to carry a minimum of weight and still to have a small factor of safety. A limit of 260 poundsless than I per cent of full load-was allowed for the radio apparatus, although the designers of the seaplanes really expected this amount to be exceeded. The apparatus was to include two complete and separate antenna systems; the radio compass; a

receiver and amplifier; two complete and independent transmitters; and sufficient electrical energy in storage and dry batteries to give continuous operation over the longest leg of the route,, and to have four hours of continuous operation on the water with the emergency set.

All this apparatus, except the spark transmitter and the coil of the radio compass, was assembled on a small table (see Fig. 1) located in one corner of the after gasoline and engineer's compartment. The general location is indicated by the X in Fig. 2.

Much to the relief of the designers and the pilots, no accident occurred during any of the tests or final flights due to the sparking of the radio apparatus in the strong gasoline fumes which were always present in the compartment, and almost equally important in the eyes of the designers and the pilots was the fact that the total weight of the radio apparatus was only 250 pounds, 12 per cent less than was allowed and 27 per cent less than had really been expected.

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The two antenna systems on the planes consisted of a trailing" antenna and a "fixed" or "skid-fin" antenna. The trailing wire was 250 feet in length and led out through the hull directly above the radio table, and thence aft to an insulator on

a brace wire, from which point it was suspended. The wire was held in a vertical position by a small, stream-lined, lead weight suspended at the end. When the planes were getting on or off the water, the wire was reeled in on a drum shown at the extreme right of Fig. 1. Connections were made to this wire as it passed through the metal core of the deck insulator. The fixed antenna consisted of a rectangular coil of wire suspended from the skid fins above the upper wing. As the brace wires in and between the wings of wire grounded, the effective range of the fixed antenna was only less than one-quarter of the range of the trailing wire. It was designed for use only when on the water but

FIG. 2.-" Official Photograph, U. S. Naval Air Service."

it served a more important purpose when the trailing wire was ripped off while flying low. Without the fixed antenna we would have been without communication during part of two legs of the flight.

Particularly important in seaplane radio installations is the radio compass. Much has been said and written about the use of the radio compass on board ships and at shore stations where ideal conditions can be realized. There, the location can be selected for its distances away from masses of steel and wires not enclosed in metal conduit. It is to be expected that the radio compass ashore and on ships will work and work well. But on board seaplanes the radio compass has been handicapped, espe

cially so on board the NC type, in which the radio compass was not easily placed or its location freely determined. By force of circumstances and by the necessity of limited space, the radio compass was installed in the only position for it, in the after compartment of the hull, amidships. There it was surrounded not only by the interior electric lighting wires and the brace wires in the hull, but also by the control wires passing back to the tail. These wires shielded and distorted the radio waves as they passed around the compass coils, and caused errors which had to be determined and allowed for. But still more disastrous to the operation of the radio compass was the fact that these wires, being connected to the common ground of the ignition system, formed an antenna to radiate and bring closer the effect of the ignition disturbances, which disturbances are of the same general nature as radio waves and have the same effect as radio waves on the compass coils and apparatus. Nevertheless, with all these difficulties, electrical remedies were applied such that the effect of the ignition system, though not eliminated, was somewhat reduced. Operating the compass revolving control cables with one hand, and the electrical controls with the other hand, listening with both ears to the signals, and watching the pointer over the coils, it required the greatest of concentration on the part of the operator to listen to the signals through the outside disturbances and it required the greatest of care to judge the intensity and thus the direction of the signals.

During the preliminary tests of the radio compass on the NC-2, an accurate range of 50 miles was attained but before the flight was taken with the NC-1, the NC-3 and the NC-4, the connections of the ignition system were changed to increase the ignition efficiency but which reduced the radio compass range to 15 miles. Unfortunately no opportunity was given, while the resources of a research laboratory were near by, to overcome these disturbances and the flight was attempted with this range. At the Azores the connections of the ignition system on the NC-4 were again changed but to no advantage to the radio compass. Shortly after leaving the Azores the auxiliary or duplicate spark plug part of the system was put out of commission by burnt out contacts, and as in this part of the ignition system had been located the cause of the greater portion of the ignition disturbances, the radio. compass functioned with its former range of 50 miles. It was

providential that this part of the ignition system did fail at this time as the NC-4 had drifted nearly forty miles off her course due to rough air and an accident to her magnetic compasses, and was practically lost. It was the radio compass which brought the NC-4 back to the line of destroyers and thence into Lisbon. Closely allied with the radio compass was the radio receiver. The radio compass itself may be considered a directive receiver, but owing to the small area that it embraces, and because all of its wire was so close to the ignition disturbances, it could not be used for long range reception. Fortunately, the shortcomings of the radio compass as a receiver were not found in the trailing wire, which made an ideal receiver. A notable case in point was a message intercepted while the NC-4 was in flight approaching the Azores. It came from the U. S. S. George Washington 1800 miles away. She was getting into Brest, France, and her radio. message which was intercepted was one reserving space on the next morning's train for Paris. This is the record for long range reception by any plane in flight.

The sensitiveness of the radio compass and the receiver and the long ranges obtained with them depended almost entirely upon the amplifier, which was of a six stage vacuum tube type. The amplifier, as will be seen in Fig. 1, was mounted on elastic supports to prevent the vibrations of the hull from affecting it. The amplifier complete was only 12" x 9" x 4" and weighed but 12 pounds. Yet this 12 pounds of apparatus amplified the incoming signals to 50 times the strength they would have been had the best previously known receiver and amplifier been used in its place.

Another interesting feature in the equipment of the NC planes was the interior telephone system. This system made it possible for any two persons in the plane to carry on a conversation at any time. A specially improved helmet was developed to hold, directly over the ears, two deep soft rubber ear cups in which were located the head telephone-receivers. The sides of these ear cups were high enough to allow each cup to be fitted to each individual person by cutting it away in places. The helmet itself was received from the factory open at the back and then was fitted to the head of each individual. The microphone, or the part into which one talked, was so constructed that the back of the diaphragm as well as its outward face was exposed to the engine